Mike makes a good point about Obama's Bill Clinton offensive--it's not immediately clear why Bill would be a riper target than his wife. I assume the thinking is that Bill has been an effective weapon and that the Obama campaign is hoping to neutralize him a bit. But maybe there's another explanation: Maybe focusing on Bill subtly reminds voters that the choice is between a return to the past--via a Clinton restoration--and the future. I think framing the race this way is Obama's best hope of capturing the nomination, and it wouldn't shock me if that were part of the thinking here.
Having said that, it's obviously not sufficient to draw Bill into the spotlight, since he remains so popular among Democrats. You have to remind voters of the things they didn't like about him, and why returning to the 90s would be a psychic drag. That's a lot trickier, but probably not impossible.
Update: Obama gets in more licks during an interview with The State newspaper in South Carolina.